You have a diesel engine and it just doesn’t seem to be running right. Maybe it’s a little rough. Noisier than usual. It doesn’t seem to have the power it had before. Even more starkly, maybe you have a diesel generator which is hard to start, and when it does, it’s making too much black smoke.
Diesel engines are more complicated than ever, especially with the arrival of the modern common rail diesel engines. So the answers to why your diesel isn’t running like it used to aren’t always simple or cut-and-dried. But if you’re going to start somewhere, you would want to make sure you’re using diesel fuel with an adequate cetane rating for your particular engine.
What Cetane Rating Really Means
Cetane, while technically a chemical, is also an index rating – an expression of how well that diesel fuel burns in a typical engine. Use a diesel fuel with too low of a rating, and the engine is going to run more roughly and without adequate power. Inadequate diesel fuel cetane rating means the diesel fuel isn’t going to combust quickly enough in that engine. So the engine doesn’t run as well as it should. It's the opposite of octane in gasoline. Gasoline with poor octane rating actually burns TOO quickly (which is why they call it pre-detonation). Diesel fuel with poor cetane rating burns too slowly.
Fortunately, there are easy ways to boost cetane in diesel fuel. All of them involve adding something to the fuel with a higher cetane rating than the fuel has by itself. Since cetane rating is actually an average of the cetane ratings of all the individual components of the diesel fuel, this makes sense. Adding more of a higher-cetane component increases the fuel’s overall cetane rating.
Recommendations for Cetane Rating
What rating should you shoot for? While 40 is the minimum rating for diesel fuel sold in the United States, 45 is the de-facto floor for the majority of diesel engines to run well. And some diesel engines prefer a rating closer to 50.
If you suspect your diesel fuel’s cetane rating isn’t high enough, there are options to boost it. Some options are better than others. If you search on the internet, you’ll find no shortage of message board posts from backyard mechanics talking about how they’ve been adding toluene to their fuel for years with great results. Or dimethyl ether. Or something else.
In truth, the industry standard for cetane improver is 2-ethyl hexyl nitrate. Not only is it effective at raising diesel cetane rating, it doesn’t cause problems in the engine or harm engine materials.
The one thing you want to watch out with cetane improvers is a similar problem that people find with octane improving additives. Lots of people sell stuff marketed as cetane improvers and octane improvers because consumers think of these as quick fixes. Unless the cetane improver you’re looking at contains 2-ethylhexylnitrate, you probably want to stay away from it. You may not know if it’s an approved additive, if it will cause damage in your engine, or (most importantly) if it will actually work.
Pay attention, also, to treat rates. Raising fuel cetane rating is not difficult to do, but you have to have enough active ingredient in the fuel. If the additive you’ve got claims to raise fuel cetane rating by 6 or 8 points, but you only have to use, say, 1 ounce per 30 gallons, you might as well put it back on the shelf. Such a small amount of additive isn't going to work. In order to raise fuel cetane rating by any meaningful amount, you probably need a minimum of 1 ounce per 10 gallons. There are no shortcuts but a cetane improver additive will do what you want if you use enough of it.
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This post was published on September 8, 2016 and was updated on February 11, 2019.